Locking screws, or bone screws, are commonly used during surgery to aid in the proper setting of bone fractures, e.g., to lock a plate to the bone across the area of the fracture in order to align and stabilize the bone fragments and to transfer the load from the bone to the plate. Bone screws are typically self-drilling and self-tapping and therefore require insertion by a drill or other power device, which typically rotate at speeds of the order of 1,000 RPM. If all the torque of the drill were continuously transmitted to the screw head, however, the screw would strip once it meets a certain resistance. As a result, a device is needed for use in a surgical setting to limit the amount of torque that is applied to the screw head.
Torque limiting devices have been developed for other applications to limit the amount of torque applied by a power tool to a workpiece. Examples of prior art devices include those disclosed in U.S. Pat. Nos. 5,437,524, 5,004,054, 4,867,019, 4,712,456, and 4,262,501. None of these devices, however, was designed for use in a surgical setting. Tests performed on certain of these prior art de rices show that they cannot be autoclaved, as required for use in the sterile environment of an operating room. For example, prior art devices typically use heavy greases or oils that cannot be sterilized or that degrade if autoclaved. In addition, prior art devices commonly use a coil spring to establish the drive connection between the drive plates. The elasticity of a coil spring may change, however, with repeated exposure to the high temperatures required for autoclaving, which affects the ability of the operator to set accurately the maximum torque exerted by the torque limiter. Also, prior art devices accumulate internally dust, grit, or rust as a result of abrasion of their moving parts, which in turn affects the ability to autoclave the device.
As the above discussion illustrates, there is a need for a torque limiting device that can be used in a surgical setting.